The present invention relates to a method for producing pulp molded articles at improved efficiency in dewatering and drying. The invention also relates to a method for producing pulp molded articles in which a prescribed portion of a wet pulp preform has a controlled water content in the steps of dewatering and drying.
Known techniques of producing pulp molded articles include the method disclosed in Japanese Patent Laid-Open No. 9-119100, which comprises immersing a papermaking mold having suction paths in a tank filled with a high-temperature pulp slurry and sucking up the pulp slurry through the suction paths to form a pulp preform. The method aims at reduction of the time required in the following dewatering step.
However, where the pulp slurry has a high temperature from the start of feeding, cost for energy is incurred, and the resulting molded article is liable to suffer from wall thickness unevenness chiefly because the pulp slurry is so flowable that the flow is disarranged where the surface layer of a wet preform is to be formed. Further, additives such as a pigment and a sizing agent sometimes fail to produce the expected effects. In particular, a pigment tends to cause color unevenness on the outer surface of a molded article which is formed in the initial stage of pulp deposition.
Production of pulp molded articles includes a dewatering step in which a wet preform is dewatered so as to increase handling properties and to shorten the time of drying in the subsequent drying step. Known dewatering methods include pressing with an elastic member or a flexible film.
In the dewatering step by pressing, the pressing force has to be increased in order to sufficiently reduce the water content of a preform, so that meshes of papermaking net may be clogged with pulp to leave a net trace on the preform, which impairs the outer appearance of the article. Further, a larger apparatus is required to apply a higher pressing force. Furthermore, mechanical dewatering has of necessity a limit in reducing the water content. Much time would be required to obtain a satisfactorily reduced water content only by dewatering.
A method of drying a wet preform by steam heating is known as disclosed in Japanese Patent Laid-Open Nos. 53-18056, 60-4320 and 9-316800. The drying method relies on heat exchange by the heat energy of steam, which is not advantageous from the standpoint of energy efficiency.
Thickness unevenness also occurs between an upper area and a lower area of the article in case of elongated pulp molded articles because pulp fibers in a pulp slurry tend to go down by gravity according to conditions.
When a preform with such a thickness unevenness is dewatered, dewatering proceeds easily in thinner-walled portions, while dewatering proceeds with more difficulty in thicker-walled portions. As dewatering proceeds, the hydrogen bond among pulp fibers in the thinner-walled portions gets closer to gain shape retention and thus, the thinner-walled portions are hard to shape as designed due to the high shape retention in the case of drying while shaping into a desired shape in the subsequent heat drying step. On the other hand, the thicker-walled portions, which are less dewatered than the thinner-walled portions, need a longer time of drying, making it difficult to shorten the time for drying the whole article.
An object of the present invention is to provide a method of producing a pulp molded article in which wall thickness unevenness can be reduced while saving the cost of energy and without impairing the effects of additives.
Another object of the present invention is to provide a method of producing a pulp molded article in which a wet pulp preform can be dewatered and dried efficiently.
Still another object of the present invention is to provide a method for producing a pulp molded article in which the water content in a prescribed portion of a wet pulp preform in dewatering and drying can be controlled to secure shapability as designed in the step of heat drying.
Yet another object of the present invention is to provide a method of producing a pulp molded article in which a pulp preform is prevented from color change due to scorching in the step of heat drying.
The above objects are accomplished by a method of producing a pulp molded article comprising:
a papermaking step in which a pulp slurry is fed to the surface of a papermaking mold having suction paths, and water contained in the pulp slurry is sucked through the suction paths whereby the pulp is deposited on the surface to form a wet preform, and
a dewatering step in which the wet preform is dewatering, wherein
the temperature of the fed pulp slurry is raised while the pulp is being deposited on the surface.
The objects. of the present invention are also accomplished by a method for producing a pulp molded article comprising:
a papermaking step in which a pulp slurry is fed into the cavity of a papermaking mold, and the cavity is evacuated toward the outer side of the papermaking mold to form a wet pulp preform on the inner wall of the mold, and
a dewatering step in which a dewatering fluid is blown into the cavity in a closed state to dewater the preform, wherein
the dewatering fluid is superheated steam, and the steam is blown in such a manner that the pressure in the cavity increases to 98 kPa[gauge] or higher, or the dewatering fluid is heated or non-heated compressed air, and the air is blown in such a manner that the pressure in the cavity increases to 196 kPa[gauge] or higher.